Converters of different types which are operated as rectifiers may be used for supplying direct current systems from three-phase current sources, in particular motor vehicle electrical systems by way of three-phase generators. In motor vehicle electrical systems, in accordance with the three-phase, four-phase, or five-phase generators typically installed here, converters in six-pulse, eight-pulse, or ten-pulse designs are typically used. The present invention is also suitable for converters for other numbers of phases or pulses, however.
When reference is made hereafter to a generator, for the sake of simplicity, this may also be an electric machine operable as a generator and a motor in this case, for example, a so-called starter generator. A converter is understood hereafter as a bridge circuit of a known type, which operates as a rectifier in the case of generator operation of the electric machine. For the sake of simplicity, also referred to hereafter as a rectifier. An arrangement made up of at least one electric machine operable as a generator and a corresponding converter operating as a rectifier is also referred to hereafter as a power supply device.
A critical operating case with corresponding power supply devices is the so-called load dump. This occurs if, in the case of a highly energized electric machine and a correspondingly high emitted current, the load on the electric machine or the converter suddenly decreases. A load dump may result from a shutdown of consumers in the connected motor vehicle electrical system or from a cable breakage.
If consumers are suddenly shut down in a motor vehicle electrical system, in particular during battery-free operation, due to the inductance of the exciter winding and the exciter field, which therefore only dissipates slowly, the electric machine may still supply more energy for up to a second than the motor vehicle electrical system is able to absorb. If this energy cannot be absorbed or cannot be absorbed completely by capacitive elements in the motor vehicle electrical system or in the converter, overvoltages and overvoltage damage may occur in components in the motor vehicle electrical system.
In the event of a cable breakage, by which the motor vehicle electrical system is disconnected from the converter, the electric machine also continues to supply energy, but a consumer is no longer connected. In comparison to the case just explained of the shutdown of consumers, consumers are therefore no longer at risk. The consumers may also still be supplied by the battery. However, the power electronics of the electric machine or the converter may be damaged in such cases by overvoltages.
In conventional (passive) converters, a certain protection of the vehicle electrical system or the power electronics of the electric machine and the converter takes place due to the converter itself, namely with the aid of the Zener diodes classically installed therein, in which the overvoltage is arrested and excess energy is converted into heat. The use of additional arresting elements is also known in this context.
However, the use of active or controlled converters is desirable in motor vehicles, inter alia, because active converters, in contrast to passive or uncontrolled converters, have low power losses during normal operation. Presently available activatable or active current control valves for active converters, for example, field-effect transistors, do not have an integrated arresting function with sufficient robustness such as conventional Zener diodes, however, and therefore cannot absorb the overvoltage. Additional protective strategies are therefore absolutely necessary in active converters.
In the event of a load dump, for example, the generator phases may be short-circuited, by briefly switching some or all current control valves of the upper or lower branch of a corresponding converter to be conductive. This is carried out in particular on the basis of an evaluation of the vehicle electrical system voltage applied at the DC voltage terminals of the converter. If this voltage exceeds a predefined upper threshold value, a corresponding short-circuit is initiated and the vehicle electrical system voltage drops. If the vehicle electrical system voltage then falls below a predefined lower threshold value, the short-circuit is canceled and the vehicle electrical system voltage increases again. The vehicle electrical system voltage therefore swings between the upper and the lower threshold values until the exciter field has abated.
An electric machine having an (at least largely) abated exciter field is also referred to hereafter as “de-energized”; an electric machine having an exciter field which is not or is only slightly abated is referred to as “energized”. When reference is made hereafter to a “phase short-circuit is initiated”, this is understood to mean that, as explained, the current control valves of the upper or the lower branch of a converter are switched to be conductive. A corresponding phase short-circuit is “canceled” when the regular active rectification is resumed again, for example, using the known pulse width modulation activation or block activation.
In the explained methods, a continuous alternation, which may no longer be ended in a conventional manner, between active rectification and phase short-circuits may take place. Since capacitively acting elements in the vehicle electrical system are no longer available in the event of a cable breakage and the capacitive elements present in the converter are comparatively small, small amounts of energy are sufficient to raise the vehicle electrical system voltage (of the remaining network which is not disconnected by the cable breakage) again in such a way that the threshold value used to initiate the phase short-circuits is exceeded. The method therefore no longer comes “to rest”, i.e., it does not enter the permanently active rectification or only does so very late. This problem is also explained below with reference to the figures.
Converters in which, in addition to an arrangement for activating a corresponding load dump reaction in the form of phase short-circuits, a voltage arresting function is provided, are also affected by this problem. Corresponding arresting circuits are configured to absorb voltage peaks before a load dump reaction in the form of phase short-circuits may be activated. A voltage arresting function induced by the arresting circuits is activated from a point in time from which the vehicle electrical system voltage or a corresponding voltage potential rises to a predefined threshold value, and is kept activated as long as the voltage potential does not drop below the threshold value. Due to the arrest, the vehicle electrical system voltage no longer increases above the threshold value, which is defined as at least temporarily safe. Such a voltage arresting function in converters, in which phase short-circuits are also used, typically includes activating the current control valves in the branches of the converter not used for the phase short-circuits and therefore establishing a conductive connection between the phase terminals connected to these current control valves and the corresponding DC voltage terminal.
The approaches known from the related art have proven, as mentioned, to not always be advantageous in particular in the event of cable breakages, so that the demand for an improved protective strategy exists for such cases.